1. Field of the Invention
The present invention relates to a cobalt compound and especially a cobalt compound successfully usable as a cobalt source for lithium-cobalt composite oxide, and a process for producing a lithium-cobalt composite oxide produced by using said cobalt compound as a cobalt source.
2. Description of the Related Art
Transition metal oxides having a hexagonl laminar crystal structure are known to allow ingress of metallic ion having an appropriate size into the crystal lattice sites and/or between the crystal lattices. Especially in case of lithium interlaminar compounds, lithium ion can be introduced into crystal lattice sites and/or between crystal lattices and the introduced lithium ion can be taken out again, under a specific potential difference. For this reason, lithium cells and lithium secondary cells using a lithium composite oxide as an electrode active material are industrially used and manufactured.
As electrode-active material, cobaltic salt of lithium (lithium cobaltate) is most typical and most effective. Although there have been made various attempts to replace the expensive cobalt with other inexpensive transition metals such as nickel, manganese or the like, no technique capable of completely replacing cobalt with other transition metal has yet been established.
Lithium cobaltate, however, has a fault that its discharging capacity decreases when charging and discharging are repeated. This decrease has been considered attributable to that repetition of charging and discharging, namely egress and ingress of lithium ion, is accompanied by contraction and expansion of crystal lattice (J. Electrochem. Soc., 139, 2091 (1992)) which causes destruction of the active material granules and elimination thereof.
With the aim of overcoming such a fault of lithium cobaltate and improving the charging-discharging characteristics thereof, a variety of attempts have been made so far.
Lithium composite oxides are generally prepared by a solid phase reaction, namely by heating a mixture of granular starting materials constituting the objective compound. Accordingly, the characteristic properties of the granular active material are much influenced by selection of starting constituents, properties of starting granular constituents and homogeneity of the mixture.
According to the prior process for producing a granular electrode active material consisting of interlaminar lithium compound, namely the so-called dry process, predetermined quantities of granular starting materials which are to form the active material are mixed together and homogenized with pulverization, and the mixture of precursors of active material thus obtained is heat-treated.
According to this prior process, it is impossible to mix the starting constituents on a molecular level and to disperse the materials uniformly even on the level of granules. Thus, the active materials produced according to such a prior process have been markedly uneven in formulation.
Apart from the dry process, a wet process or a process of preparing a solution of active material-constituting elements and depositing salts of the elements to prepare a uniform mixture of granular precursors has also been studied. According to the conventional wet process, the elements are deposited or precipitated in the form of oxalates, and the oxalates are washed with water, dried and calcined to obtain a granular active material. However, water-solubility of oxalate is greatly different from an element to another, so that formulation of final product becomes deviating from the starting formulation in the course of washing.
Further, as yet another process, precipitation of the elements in the form of hydroxides or carbonates has also been studied. This process, however, is similar to the oxalate process in that formulation of final product unavoidably deviates from the initial one in the course of washing and this process has an additional fault that coarse granules are formed during calcination.
Apart from the processes mentioned above, there has also been studied a process of converting a sol consisting of alkoxides of lithium and other prescribed transition metals constituting an electrode active material to a gel according to the sol-gel conversion method, and calcining the gel to obtain an active material (WO92/18425). Since this method makes it possible to obtain a lithium cobaltate in which the constituents are uniformly arranged on a molecular level, this process is expected to give an active material highly resistant to the contraction and expansion brought about by egress and ingress of lithium ion and capable of manifesting excellent charging-discharging characteristics. However, this process is difficult to adopt industrially, because of the extreme expensiveness of alkoxides.
Since the material forming the matrix releasing and accepting lithium ion is a cobalt compound, many studies have so far been conducted on the cobalt source.
For instance, a method of using a cobalt oxide prepared by heat-treatment of spherical or ellipsoidal cobalt hydroxides prepared by crystallization method has been proposed (JP-A 54888/1993). However, the active material prepared according to this technique cannot be considered especially excellent in charging-discharging characteristics.
There has also been proposed a method for preparing a lithium cobaltate of specific form which uses a Co.sub.3 O.sub.4 prepared by heat-treatment of specific cobalt hydroxide as a cobalt source in order to reproduce the form of the starting cobalt hydroxide (JP-A 022693/1997). However, the cobalt hydroxide used therein is nothing other than quite usual one, so that the lithium cobaltate produced therefrom has no characteristics to be especially differentiated from usual characteristics. That is to say, this technique cannot be said to offer a method for essential solution of the problem. Additionally saying, the description that a product obtained by heat-treating a cobalt hydroxide at a temperature higher than the decomposition temperature thereof to form Co.sub.3 O.sub.4 and then calcining the thus obtained Co.sub.3 O.sub.4 as a cobalt source at a temperature of 900.degree. C. reproduces the form of the starting cobalt hydroxide is quite unnatural.
Further, an attempt to produce a lithium cobaltate from a trivalent cobalt compound originally having the form of hexagonal crystal system at a low temperature by the technique of ion exchange has also been studied (Solid State Ionics, 84, 169 (1996)). This technique, however, has a fault that it requires a complicated procedure of pelletizing the above-mentioned cobalt compound together with twice its equivalent quantity of lithium compound, adding an approximately same quantity, as that of the pellet, of water to the pellet, and allowing the mixture to stand for 5 days under an elevated pressure of at least 6 atmospheres. Furthermore, this technique has a fault that a usable electrode active material is obtained only after washing away the excessive lithium compound and heat-treating the washed material at a temperature of 250.degree. C. or above for one day.